fix accessing undefined variable (see #2806)

Description
-----------

| Q                | A
| -----------------| ---
| Fixed issues     | Fixes #...
| Docs PR or issue | contao/docs#...

Sigh, another one for PHP8.

Fixes 
```
Warning: Undefined array key "breadcrumb"
```
after clicking on the articles selection in the page tree because the `sorting` array does not contain `breadcrumb`.

Commits
-------

36dcf5e fix accessing undefined variable

core-bundle/src/Resources/contao/classes/Backend.php

@@ -894,7 +894,7 @@ public static function addPagesBreadcrumb($strKey='tl_page_node')
 		$arrLinks = array_reverse($arrLinks);
 
 		// Insert breadcrumb menu
-		$GLOBALS['TL_DCA']['tl_page']['list']['sorting']['breadcrumb'] .= '
+		$GLOBALS['TL_DCA']['tl_page']['list']['sorting']['breadcrumb'] = ($GLOBALS['TL_DCA']['tl_page']['list']['sorting']['breadcrumb'] ?? '') . '
 
 <nav aria-label="' . $GLOBALS['TL_LANG']['MSC']['breadcrumbMenu'] . '">
   <ul id="tl_breadcrumb">
@@ -1068,7 +1068,7 @@ public static function addFilesBreadcrumb($strKey='tl_files_node')
 		$GLOBALS['TL_DCA']['tl_files']['list']['sorting']['root'] = array($strNode);
 
 		// Insert breadcrumb menu
-		$GLOBALS['TL_DCA']['tl_files']['list']['sorting']['breadcrumb'] .= '
+		$GLOBALS['TL_DCA']['tl_files']['list']['sorting']['breadcrumb'] = ($GLOBALS['TL_DCA']['tl_files']['list']['sorting']['breadcrumb'] ?? '') . '
 
 <nav aria-label="' . $GLOBALS['TL_LANG']['MSC']['breadcrumbMenu'] . '">
   <ul id="tl_breadcrumb">